1. Field of the Invention
The present invention relates to a light-emitting device using gallium nitride compound semiconductor whose luminous efficiency is improved. Especially, the present invention relates to the device which emits the ultraviolet ray.
2. Description of the Related Art
A conventional light-emitting device, which have layers made of gallium nitride compound semiconductor laminated on a substrate, is known to have the following structure. The device has a sapphire substrate, and on the sapphire substrate the following layers are formed sequentially: a buffer layer made of aluminum nitride (AlN); an n-cladding and/or an n-contact layer of high carrier concentration, which is made of a silicon (Si) doped GaN of n-type conduction; an emission layer having a multi quantum-well (MQW) structure, in which a barrier layer made of GaN and a well layer made of InGaN are laminated alternately; a p-cladding layer made of magnesium (Mg) doped AlGaN of p-type conduction; and a p-contact layer made of magnesium (Mg) doped GaN of p-type conduction.
And a conventional light-emitting device using gallium nitride compound semiconductor which emits the ultraviolet ray is known to have an emission layer made of InGaN or AlGaN. The device having an emission layer made of InGaN can obtain an ultraviolet ray having a wavelength of lower than 380 nm, which is emitted from band to band, when a composition ratio of indium (In) is less than 5.5%. The device having an emission layer made of AlGaN can obtain an ultraviolet ray having a wavelength of 380 nm, which is emitted by a pair of donor and acceptor, when a composition ratio of aluminum (Al) is about 16% and the emission layer is doped with zinc (Zn) and silicon (Si).
However, a problem persists in luminous efficiency. In the conventional light-emitting devices using gallium nitride compound semiconductor, conditions for emitting light are not always optimized. Therefore, further improvement has been required, as presently appreciated by the present inventors.
An object of the present invention is to improve luminous efficiency of a light-emitting device using gallium nitride compound semiconductor.
To achieve the above object, a first aspect of the present invention is to obtain a light-emitting device using gallium nitride semiconductor comprising an emission layer with a multi quantum-well (MQW) structure, in which a barrier layer and a well layer are formed alternately. The barrier layer is made of AlxGa1xe2x88x92xN (0 less than xxe2x89xa60.18).
The second aspect of the present invention is to form the well layer made of InyGa1xe2x88x92yN (0xe2x89xa6yxe2x89xa60.1).
The third aspect of the present invention is to form the barrier layer to have a thickness from 2 nm to 10 nm.
The fourth aspect of the present invention is to form the barrier layer to have a thickness from 3 nm to 8 nm.
The fifth aspect of the present invention is to design a luminous wavelength in the ultraviolet ray region.
The sixth aspect of the present invention is to obtain a light-emitting device using gallium nitride compound semiconductor comprising an emission layer with a multi quantum-well (MQW) structure, in which a barrier layer and a well layer are formed alternately, and an n-layer made of an impurity-doped AlGa1xe2x88x92xN (0 less than xxe2x89xa60.06).
The seventh aspect of the present invention is to form a strain relaxation layer made of InyGa1xe2x88x92yN (0.02xe2x89xa6yxe2x89xa60.04) which is formed under the n-layer.
The eighth aspect of the present invention is to form the n-layer to have a thickness from 50 nm to 300 nm.
The ninth aspect of the present invention is to form the n-layer to have a thickness from 150 nm to 250 nm.
The tenth aspect of the present invention is to design a luminous wavelength to be in the ultraviolet ray range.
The eleventh aspect of the present invention is to obtain a light-emitting device using gallium nitride compound semiconductor comprising an emission layer with a multi quantum-well (MQW) structure, in which a barrier layer and a well layer are formed alternately, a p-layer, and an n-layer. The emission layer is sandwiched by the p-layer and the n-layer, and a ratio of an electron concentration of the n-layer to a hole concentration of the p-layer (electron/hole) is from 0.5 to 2.0.
The twelfth aspect of the present invention is to obtain a light-emitting device using gallium nitride compound semiconductor comprising an emission layer with a multi quantum-well (MQW) structure, in which a barrier layer and a well layer are formed alternately, a p-layer, and an n-layer. The emission layer is sandwiched by the p-layer and the n-layer, and a ratio of an electron concentration of the n-layer to a hole concentration of the p-layer (electron/hole) is from 0.7 to 1.43.
The thirteenth aspect of the present invention is to obtain a light-emitting device using gallium nitride compound semiconductor comprising an emission layer with a multi quantum-well (MQW) structure, in which a barrier layer and a well layer are formed alternately, a p-layer, and an n-layer. The emission layer is sandwiched by the p-layer and the n-layer, and a ratio of an electron concentration of the n-layer to a hole concentration of the p-layer (electron/hole) is from 0.8 to 1.25.
The fourteenth aspect of the present invention is to design a luminous wavelength in the ultraviolet ray range.
With respect to a gallium nitride compound semiconductor which satisfies the formula AlxGa1xe2x88x92xxe2x88x92yInyN, the larger a composition ratio x of aluminum (Al), is, the larger a band gap energy becomes, and the larger a composition ratio y of indium (In) is, the smaller the band gap energy becomes. With respect to a light-emission device using gallium nitride compound semiconductor which has an emission layer with a multi quantum-well (MQW) structure, an energy barrier between a well layer and a barrier layer becomes larger when the barrier layer is made of AlxGa1xe2x88x92xN. A luminous intensity of the device is strongly related to a composition ratio x of aluminum (Al) in AlxGa1xe2x88x92xN barrier layer. Various samples of a barrier layer made of AlxGa1xe2x88x92xN, each having a different composition ratio x of aluminum (Al), are formed and the electroluminescence (EL) luminous intensity is measured. FIG. 2 illustrates a graph of the electroluminescence (EL) luminous intensity. As shown in FIG. 2, the luminous intensity of the light-emitting device becomes larger in accordance with the composition ratio of aluminum (Al). The composition ratio x should be preferably in the range of 0.06xe2x89xa6xxe2x89xa60.18. When x, or a composition ratio of aluminum (Al), is smaller than 0.06, an effect for mixing aluminum (Al) in the barrier layer is small. When x is larger than 0.18, a lattice matching of the barrier layer becomes worse and as a result luminous intensity is lowered.
Samples of a light-emitting device having a well layer made of InyGa1xe2x88x92yN which has a smaller band gap are formed. When y, or a composition ratio of indium (In), is smaller than 0.1, a crystallization of the well layer becomes worse, and the device cannot have a large luminous intensity.
Various samples of a barrier layer each having a different thickness are formed. FIG. 3 illustrates the electroluminescence (EL) luminous intensity of the light-emitting device having the barrier layer made of AlxGa1xe2x88x92xN. As shown in FIG. 3, the thickness of the barrier layer should be preferably in the range of 2 nm to 10 nm, more preferably 3 nm to 8 nm.
When an n-cladding layer which contacts to the emission layer is made of AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa60.06), holes in the emission layer is prevented from leaking to the lower n-layer side through the n-cladding layer. Also, a lattice mismatching of the emission layer which is grown on the n-cladding layer can be relaxed and as a result a crystallization of the emission layer is improved. Accordingly, a luminous efficiency of the light-emitting device can be improved.
A luminous intensity of the light-emitting device is strongly related to a composition ratio x of aluminum (Al) in AlxGa1xe2x88x92xN n-cladding layer. Various samples of an n-cladding layer made of AlxGa1xe2x88x92xN, each having a different composition ratio x of aluminum (Al), are formed and the electroluminescence (EL) luminous intensity is measured. FIG. 5 illustrates a graph of the electroluminescence (EL) luminous intensity. As shown in FIG. 5, the luminous intensity of the light-emitting device becomes larger in accordance with the composition ratio of aluminum (Al). And when the composition ratio x is around 0.05, luminous intensity of the device shows its peak. The composition ratio x should be preferably in the range of 0.03xe2x89xa6xxe2x89xa60.06. When x, or a composition ratio of aluminum (Al), is smaller than 0.03, the device becomes just like a device without an n-cladding layer and holes leak to the lower n-layer side through the n-cladding layer. When x is larger than 0.06, a crystallization of the emission layer is lowered because of too much aluminum (Al) existing in the n-cladding layer, and as a result the luminous intensity of the device is lowered. various samples of an n-cladding layer each having a different thickness are formed. FIG. 6 illustrates the electroluminescence (EL) luminous intensity of the light-emitting device having the n-cladding layer made of AlxGa1xe2x88x92xN. As shown in FIG. 6, the luminous intensity of the device shows its peak when the thickness of the n-cladding layer is around 200 nm. The thickness of the n-cladding layer should be preferably in the range of 50 nm to 300 nm, more preferably 150 nm to 250 nm.
With respect to a light-emitting device using gallium nitride compound semiconductor which has a double-hetero junction structure, forming an n-type layer is easier than forming a p-type layer. A hole concentration of the p-type layer is smaller than an electron concentration of the n-type layer. FIGS. 8 and 9 illustrate graphs of the electroluminescence (EL) luminous intensity of the light-emitting device when each electron concentrations of an n-cladding layer and an n-contact layer is varied in order that a ratio of the electron concentration of each n-type layers, the n-cladding layer and the n-contact layer to a hole concentration of each p-type layers, a p-cladding layer and a p-contact layer, respectively, approximates to 1. Here a hole concentration of the p-cladding layer and the p-contact layer is 2xc3x971017/cm3 and 7xc3x971017/cm3, respectively.
A luminous intensity of the light-emitting device is strongly related to an electron concentration of Al0.05Ga0.95N n-cladding layer. Various samples of an n-cladding layer made of Al0.05Ga0.95N, each having a different electron concentration, are formed and the electroluminescence (EL) luminous intensity is measured. FIG. 8 illustrates a graph of the electroluminescence (EL) luminous intensity. As shown in FIG. 8, the luminous intensity of the light-emitting device shows its peak when the electron concentration of the n-cladding layer is around 8xc3x971017/cm3.
Also, the luminous intensity of the light-emitting device is strongly related to an electron concentration of GaN n-contact layer. Various samples of an n-contact layer made of GaN, each having a different electron concentration, are formed and the electroluminescence (EL) luminous intensity is measured. FIG. 9 illustrates a graph of the electroluminescence (EL) luminous intensity. As shown in FIG. 9, the luminous intensity of the light-emitting device becomes larger in accordance that the electron concentration of GaN n-cladding layer becomes 1.1xc3x971018/cm3, 8xc3x971017/cm3, and 4xc3x971017/cm3. This proves that a recombination of electrons and holes occurs at the inside of the emission layer. In short, when an electron concentration of the n-cladding layer or the n-contact layer is larger than a hole concentration of the p-cladding layer or the p-contact layer, electrons tend to recombine with holes at the p-contact or the p-cladding layer side from the emission layer. And if a recombination of electrons and holes which does not emit lights increases under this condition, it is considered that balancing a hole concentration of the p-cladding or the p-contacting layer and an electron concentration of the n-cladding or the n-contact layer is effective for decreasing the non-emissive recombination of electrons and holes.
Here the n-cladding layer made of GaN needs to have an electron concentration of at least 1xc3x971017/cm3 in order to form an electrode, inject electrons and drive the light-emitting device.